Silver-cadmium oxide electrical contact materials

ABSTRACT

A SILVER LAYER IS APPLIED TO SILVER-CADMIUM ALLOYS BY FIRST OXIDIZING A PORTION OF A SILVER-CADMIUM ALLOY, APPLYING A BONDING LAYER, AND THEN THE SILVER-CADMIUM ALLOY IS COMPLETELY OXIDIZED.

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5' lNVIENTOR FREDRIK o. HAARBYE #www ATTORNE May 30, 1972 F. 0I HAARBYE 3,5%;428

SHJVURUUNIIUM OXID' ELECTRICAL CONTACT MATERIALS 2 Sheets-Sheet 2 Filed March 9, 1970.

I PROVIDE l Aq-Cd MATERIAL l I6\ INTERNALLY OxIOIZE M M .H M A PORTION OE Ag-Cd CLEAN STRIP IMvH// IF NECESSARY f 'Il f' l I 4x APPLY SILVER IFI@ 5 BACKINO 5.\ I8," ROLL TO .TT-fw g I APPROXIMATE -SIZE ..;..-I ";II`-.j"A fl EIB' COMPLETE v I INTERNAL 1 OXIDATION ROLL TO FINAL SIZE TTM 4T 1PM- 2 INVENTOR FR EDR K O. H AARBYE EIY Si Uw/VO ATTORNEY 3,666,428 Patented May 30, 1972 3,666,428 SILVER-CADMIUM OXIDE ELECTRICAL CONTACT MATERIALS Frederick O. Haarbye, Indianapolis, Ind., assignor to P. R. Mallory 8: Co. Inc., Indianapolis, Ind. Continuation-impart of appliccation Ser. No. 722,956, Apr. 22, 1968. This application Mar. 9, 1970,

Ser. No. 17,456

Int. Cl. C22c 5/ 00; H01h I 02 U.S. Cl. 29-195 8 Claims ABSTRACT 0F THE DISCLOSURE A silver layer is applied to silver-cadmium alloys by 4first oxidizing a portion of a silver-cadmium alloy, applying a bonding layer, and then the silver-cadmium alloy is completely oxidized.

This is a continuation-in-part of application Ser. No. 722,956, led Apr. 22, 1968, now abandoned.

Silver-cadmium oxide materials are being increasingly used as electrical contact materials in many electrical applications. Such material substantially eliminates the tendency for sticking in make and break applications and has a low resistance for better current carrying capacity. When being used as electrical contacts, the material is usually fabricated in the form of strips which are subsequently attached to a suitable backing material such as stainless steel, for example, to form the electrical contact. Such backing material forms the shank of the contact. Normally, the attachment is done by welding or brazing. However, it has been found that silver-cadmium oxide contact materials are diflicult to attach directly to the stainless steel backing material through a brazing material. Silver-cadmium oxide material is, therefore, rst backed with a layer of silver. Such silver backing then provides a surface to which a brazing means can be applied for attaching the silver-cadmium oxide material to the stainless steel.

In the drawings:

FIG. 1 is a photomicrograph at 300X showing the cadmium oxide layer obtained according to prior art bonding techniques;

FIG. 2 is a block diagram showing the various steps used in carrying out the process of the present invention;

FIG. 3 is a cross-section of a silver-cadmium material showing its conditions at several steps of the process;

FIG. 4 is a schematic illustration of the microstructure obtained with the process of the present invention; and

FIG. 5 is a photomicrograph at 300X of the microstructure obtained with the process of the present invention.

The application of silver backing to the silver-cadmium oxide material as described above is not, however, without its problems. For example, as shown in FIG. l, a heavy layer of cadmium oxide 14 is generally encountered near or at the interface of backing 12 and the main body of silver-cadmium oxide 10. Such layer results in impairment of the attachment of the contact material to other materials such as stainless steel, because of the low mechanical strength of the cadmium oxide. Also, in applying the silver backing, it is highly desirable, because of ease of handling, that the silver be applied to a relatively thick material, about 1% inch or more. Because of this, the silver is preferably applied to a silver-cadmium alloy rather than a silver-cadmium oxide alloy since it is difficult to reduce the thickness of silver-cadmium oxide without cracking. However, when applying the silver backing to a silver-cadmium alloy there is a strong tendency for cadmium to migrate from the main body of silver-cadmium alloy toward the surface to which the silver is being applied. Prior to attachment there is a slight gap between the backing and the unoxidized silver-cadmium alloy. Thus the above-noted cadmium oxide layer 14 is obtained.

The present invention is concerned with the provision of an electrical contact material comprising a silver-cadmium oxide body having a layer of a material which is highly thermally and electrically conductive and which is readily wettable for subsequent brazing applied thereto and has as one of its objects the provision of such a material which will not be rich in cadmium oxide at the interface between the layer and the silver-cadmium oxide material.

Another object of the invention is the provision of such an electrical contact wherein the layer is silver, copper, aluminum or gold.

Another object of the invention is to provide a method of forming an electrical Contact material comprising a silver-cadmium oxide body having a silver layer bonded thereto.

Another object of the invention is to provide a method of fabricating a silver-cadmium oxide electrical contact material with a silver layer bonded thereto wherein a barrier is established at or near the surface to which the silver layer is to be bonded.

Yet another object of the invention is to provide a method of forming a silver-cadmium oxide electrical contact material having a silver layer bonded thereto wherein the silver-cadmium alloy is internally oxidized at or near the surface to which the silver layer is to be applied to form the barrier.

A further object of the invention is to provide a method of forming a silver-cadmium oxide electrical contact material having a silver layer bonded thereto wherein the silver layer is applied after a portion of the silver-cadmium has been internally oxidized.

Still another object of the invention is to provide a method of forming a silver-cadmium oxide electrical contact material having a silver layer bonded thereto wherein a portion of a silver-cadmium alloy is internally oxidized prior to application of the silver layer, the silver layer applied, and the oxidation of substantially the entire material is completed.

Another object of the invention is to provide an electrical contact material comprising a Ibody of silver-cadmium oxide with a layer of silver bonded thereto, the body having a zone of barrier material consisting of silver-cadmium oxide at or near the surface of the body having the silver layer.

These and other objects of the invention and the nature thereof will become apparent from the following description.

Generally speaking, the objects of the invention are accomplished by providing a method of forming an electrical contact material comprising a body of silver-cadmium oxide at least one layer of a high electrically and thermally conductive material which is readily Wettable for subsequent brazng bonded thereto, wherein the layer that is bonded to the body is applied after a portion of a silver-cadmium alloy material is first internally oxidized to form a barrier of silver-cadmium oxide, the alloy being oxidized at or near the surface of the material to which the layer is to be bonded. After the layer has been applied, the internal oxidation of the silver-cadmium material is then completed. The method of the invention results in an electrical contact material comprising a body of silver and cadmium oxide having a layer of a high electrically and thermally conductive and readily wettable material bonded to the silver-cadmium oxide body with substantially no massive cadmium oxide at the body layer interface.

In prior art silver-cadmium oxide contact material having a silver layer bonded thereto, there is usually a heavy deposit of cadimum oxide such as 14 in FIG. 1 at the interface between the silver layer 12 and the silver cadmium oxide material 10. With such a microstructure it has been found that even with the silver layer acting as a wetting means for subsequent brazing, it is very difiicult to attach the silver-cadmium oxide electrical contact material to a backing material such as stainless steel. More specifically, with the heavy concentration of cadmium oxide at the braze interface when no silver layer is used, or at the interface between the silver-cadmium oxide body and the silver layer `when a silver layer is used, there is obtained a bond low in strength because of the low physical strength of the cadimum oxide layer. This diliiculty has been substantially eliminated by the process of the present invention.

With reference to FIGS. 2 and 3, there is shown the particular steps used in carrying out the method of the invention. As shown, there is provided, in step 1, a silver-cadmium material 18. The material may take several forms such as sheets, wire strips, etc. Various proportions of silver and cadmium may be used in carrying out the steps of the invention. Generally speaking, in the practice of the invention there may be from 2.0% cadmium by weight of the total silver-cadmium material 18 to about 15% by weight, the balance being substantially silver. Exemplary percentages of cadmium by weight within this range include 2.2% cadmium, 4.5% cadmium, 9% cadmium, 12% cadmium, 13.4% cadmium and 15% cadmium, with the balance being silver. The silver-cadmium material is then, as shown in step 2, internally oxidized such that a portion of the silver-cadmium is oxidized to form relatively thin zones of silver-cadmium oxide 16 and 16. Zones 16 and 16 act as barriers and need to be thick enough to prevent cadmium from the balance of the material 18 from penetrating the barrier when a layer of a highly thermally and electronically conductive and readily wettable material for brazing such as silver, copper, aluminum or gold is applied to the surface. 'Ihe oxidation is, in general, carried out by passing the material through a furnace in an oxidizing atmosphere such as air or pure oxygen and heating the ma` terial to a maximum temperature of about 850 C. and preferably about 700 to about 825 C., most preferably about 800 to 825 C.

`Generally speaking, the time needed to form the zone 16 is dependent upon the amount of cadmium in the material and the needed thickness of the layer. For a material having from 2-l5% cadmium by weight, it has been found that the oxidation time may be for example from about to 30 minutes depending upon temperature, preferably about to 25 minutes in order to achieve a satisfactory thickness, for example of about .001 may be used. In passing the material through the furnace no particular care need be taken to assure that there will be a cadmium oxide layer only at one surface. Thus, the oxygen from the oxidizing atmosphere will penetrate all sides and ends of the material and will therefore form the other zone 16'. The point is, however, that only a suflicient amount of oxidation occurs so as to provide a relatively thin zone 16 and 16 to act as a barrier material.

As is next shown in step 3, if there should be any minor amounts of cadmium oxide stain or tarnish present on the surface of the material, the material may be cleaned. Such cleaning may he done by mechanical brushing or by the use of a suitable silver cleaner such as hydrochloric acid, for example.

As is shown in step 4, a bonding layer is then applied to the material. As previously noted, silver, copper, aluminum, gold, alloys thereof or other readily wettable material may also be used. The layer is usually applied by a hot roll bonding Iprocess. Prior to the hot rolling step, a sandwich of the material with the bonding layer is first prepared. As shown in FIG. 3, a strip 20 for example of silver is laid over the surface to which the layer is to be applied. In order to insure adequate brazing of the silver-cadmium oxide material to the backing material used to form electrical contacts, the thickness of the strip 20 should be preferably from about 5-10% of the thickness of the silver-cadmium material 18, a minimum of .005 at nal size. This sandwich is then preheated in a furnace at a temperature of from about 700 to about 825 C. for a period of about 5 to 25 minutes in order to not complete the oxidation of the silvercadmium material in an oxidizing atmosphere. Such time, and the temperature to which the sandwich is heated, is dependent upon the rolling characteristics of the material which is partly dependent ufpon the amount of cadmium present. It is during the preheating step that the barrier zone 16 becomes most important. It prevents migration (diffusion) of cadmium from the silver-cadmium material 18 to the silver-cadmium silver layer interface. Such migration will cause formation of the heavy layer of cadmium oxide at the silver layer interface, as is commonly found in prior art materials, and which is shown at 14 in FIG. 1 when the material is completely oxidized. After the sandwich has been prepared, the sandwich is fed through suitable rolling means that have been preheated to a temperature of about 300 C. to 450 C. so as to roll and pressure weld the silver strip 20 to the material 18. Upon passing through the roll, the thickness of the sandwich will be substantially reduced, there being about a 10-30% reduction with the optimum being about a 15 to 25% reduction in the thickness. AS indicated in step 5, the sandwich can be subsequently rolled a number of times to reduce the thickness of the sandwich to approximately the final desired size. Such rolling may be either hot rolled, or cold rolled at ambient temperatures.

As indicated in step 6 and FIG. 3, after the rolling is completed, the silver-cadmium material, a portion of which has been oxidized, and having the backing layer bonded thereto is internally oxidized to complete the oxidation such that the entire silver-cadmiuml material 18 is substantially completely oxidized. Such oxidation is carried out in the manner as was done in the initial oxidation step of step 2. The oxygen will penetrate the silver layer 20 that has been bonded to the body, but there will be substantially no penetration of cadmium from the silver-cadmium material 18 to the interface between the backing layer and the material 18 due to the barrier of silver-cadmium oxide 16. The amount of time sufficient to complete the oxidation will be dependent upon the lamount of cadmium present in the original silver-cadmium material and the thickness of the material. As is indicated in step 7, after the material has been completely oxidized, the material may then be rolled to final size if necessary.

With particular reference to FIGS. 4 and 5, the final structure is shown. The structure 18' will comprise a body of silver-cadmium oxide 18", a silver layer 20 bonded to the silver-cadmium oxide body 18" with substantially no massive cadmium oxide present at the interface. Additionally there will be present region 24 containing fine cadmium oxide particles, covered by a relatively thin zone 16 of silver-cadmium oxide of slightly coarser cadmium oxide particle size, unless the entire body has been oxidized in the first oxidation step, as will be the case with thin silver-cadmium material below about 1/a at the bonding stage. After being substantially completely oxidized, the amount of silver-cadmium oxide present in the portions 18" and 16 will consist of from about 2 to 17% by weight of the material in these portions with the balance being substantially silver. Exemplary percentages of cadmium oxide within this range include 2.6%, 5%, 10%, 13.4%, 15% and 16.8% with the balance in each case being essentially silver.

This material can then be brazed to a suitable backing material such as stainless steel from which an electrical contact can be made.

As an example of the present invention, silver layers may be bonded to each of 0.3 inch thick strips of silvercadmium alloys having 9% and 14.3% cadmium by weight respectively of the strips in the following manner:

The strips may be first oxidized in an air atmosphere at a temperature of from SUO-825 C. for a period of from 15-20 minutes to achieve a barrier zone 16 of at least .001 inch in thickness. To each of these partially oxidized strips a silver layer .033 inch may be applied to one-of the surfaces thereof by first preparing a sandwich of the strip and the layer, and then hot roll bonding the layer at a temperature of from 775 to 825 C. The strips with the silver layer may be reduced .066 inch to achieve a 20% reduction in thickness. The rolled stnip with the silver layer may then be reduced to a final thickness of .125 inch by rolling at ambient temperatures.

The oxidation of the silver-cadmium strips with the silver layer may then be completed by oxidizing the strip in an air atmosphere for a period of from 70-75 hours. The final composition of the contact material will be silver-cadmium oxide strips having about 10 and 15% cadmium oxide by weight, each having a silver layer of a thickness of about .0125 inch bonded to each strip.

Similar procedures may be used to form silver-cadmium oxide strips with a silver layer bonded thereto for the other percentages of silver-cadmium oxide.

Thus there is described an electrical contact material which can be attached, with cutter strength of attachment, to a backing material such as stainless steel to form the contact.

What is claimed is:

1. Electrical contact material comprising:

a body of silver and cadmium oxide, a layer of high electrically and thermally conductive and readily wettable material bonded to said body;

the interface between said body and said layer being substantially free of massive cadmium oxide;

said body adjacent said interface comprising a first region having a finer cadmium oxide particle size than the remainder of said body, and a second region which defines said interface, which second region has a coarser particle size than said first region.

2. An electrical contact material according to claim 1, wherein said layer of high electrically and thermally conductive and readily wettable material is taken from the group consisting of silver, copper, aluminum, gold and alloys thereof.

3. An electrical contact material according to claim 2, wherein said layer of high electrically and thermally conductive and readily wettable material is silver.

4. An electrical contact material according to claim 1, wherein the amount of cadmium oxide in the silver-cadmium oxide material is from about 2-17% by weight, the balance being essentially silver.

5. An electrical contact material according to claim 1, wherein the thickness of said layer is from about 5-10% of the -thickness of the silver-cadmium oxide material.

6. An electrical contact material according to claim 1, wherein said body is in the form of a strip.

7. An electrical contact material according to claim 1, wherein said body is brazed to a backing material.

8. An electrical contact material according to claim 7,

wherein said backing material is stainless steel.

References Cited UNITED STATES PATENTS 2,673,167 3/1954 Vines 75--173 R X 2,539,298 1/1951 Doty et al. 75-173 R X 3,472,654 10/1969 Comey et al. 75-173 R L. DEWAYNE RUTLEDGE, Primary lExaminer E. L. WEISE, Assistant Examiner U.S. C1. X.R. 

